Reliable quantum communication and/or processing links between modules are a necessary building block for various quantum processing architectures. Here we consider a spin-chain system with alternating strength couplings and containing three defects, which impose three domain walls between topologically distinct regions of the chain. We show that—in addition to its useful, high-fidelity, quantum state transfer properties—an entangling protocol can be implemented in this system, with optional localization and storage of the entangled states. We demonstrate both numerically and analytically that, given a suitable initial product-state injection, the natural dynamics of the system produces a maximally entangled state at a given time. We present detailed investigations of the effects of fabrication errors, analyzing random static disorder both in the diagonal and off-diagonal terms of the system Hamiltonian. Our results show that the entangled state formation is very robust against perturbations of up to $\sim 10\%$ the weaker chain coupling, and also robust against timing injection errors. We propose a further protocol, which manipulates the chain in order to localize and store each of the entangled qubits. The engineering of a system with such characteristics would thus provide a useful device for quantum information processing tasks involving the creation and storage of entangled resources.

By theoretical calculation, we demonstrate the possibility to control and partially suppress the Coulomb explosion of ${\mathrm{N}}_2$ molecules after core-level photoionization by an x-ray laser and subsequent Auger decay. This is achieved by means of a femtosecond infrared laser pulse interacting with the ${\mathrm{N}}_2{}^{2+}$ dication produced by the x-ray pulse. Suppression of molecular fragmentation requires few-femtosecond IR pulses interacting with the system either during or shortly after the arrival of the x-ray pulse. The IR pulse suppresses fragmentation mostly by optically coupling the electronic routes to ultrafast molecular dissociation with electronic channels able to support long-lived vibrational resonances. The effect is strongly dependent on the orientation of the molecule with respect to the polarization axis of the IR field. Our calculations are motivated by x-ray pump–IR probe experiments performed at an x-ray free-electron laser [J. M. Glownia et al., Opt. Express 18, 17620 (2010)], where only enhancement of ${\mathrm{N}}_2{}^{2+}$ fragmentation as a function of the pump-probe delay time was reported. The opposite effect reported here becomes apparent when the various electronic channels are considered separately. In practice, this corresponds to a coincident measurement of the energy of the ejected Auger electron.

The photoionization of model neon atoms carrying different angular momenta in strong laser fields is studied using several different theoretical methods, such as the time-dependent Schrödinger simulation, classical trajectory Monte Carlo simulation, quantum trajectory Monte Carlo simulation, and backward propagation. The tunneling exit, time-dependent tunneling rate, and photoelectron momentum distribution at tunneling are explored by cross comparing the photoelectron momentum distributions calculated with these methods. When the electron and laser field counter rotate, the tunneling exit is more close to the nucleus, resulting in a larger tunneling ionization rate. Different transverse momenta at tunneling exits induce different ultimate photoelectron momentum distributions.

We propose and study systems of coupled atomic wires in a perpendicular synthetic magnetic field as a platform to realize exotic phases of quantum matter. This includes (fractional) quantum Hall states in arrays of many wires inspired by the pioneering work [C. L. Kane et al., Phys. Rev. Lett. 88, 036401 (2002)], as well as Meissner phases and vortex phases in double wires. With one continuous and one discrete spatial dimension, the proposed setup naturally complements recently realized discrete counterparts, i.e., the Harper-Hofstadter model and the two-leg flux ladder, respectively. We present both an in-depth theoretical study and a detailed experimental proposal to make the unique properties of the semicontinuous Harper-Hofstadter model accessible with cold-atom experiments. For the minimal setup of a double wire, we explore how a subwavelength spacing of the wires can be implemented. This construction increases the relevant energy scales by at least an order of magnitude compared to ordinary optical lattices, thus rendering subtle many-body phenomena such as Lifshitz transitions in Fermi gases observable in an experimentally realistic parameter regime. For arrays of many wires, we discuss the emergence of Chern bands with readily tunable flatness of the dispersion and show how fractional quantum Hall states can be stabilized in such systems. Using for the creation of optical potentials Laguerre-Gauss beams that carry orbital angular momentum, we detail how the coupled atomic wire setups can be realized in nonplanar geometries such as cylinders, disks, and tori.

We report Dirac monopoles with a polar-core vortex induced by spin-orbit coupling in ferromagnetic Bose-Einstein condensates, which are attached to two nodal vortex lines along the vertical axis. These monopoles are more stable in the time scale of experiment and can be detected through directly imaging vortex lines. When the strength of spin-orbit coupling increases, Dirac monopoles with a vortex can be transformed into those with a square lattice. In the presence of spin-orbit coupling, increasing the strength of the interaction can induce a cyclic phase transition from Dirac monopoles with a polar-core vortex to those with a Mermin-Ho vortex. The spin-orbit coupled Bose-Einstein condensates not only provide a unique platform for investigating exotic monopoles and relevant phase transitions but also can hold stable monopoles even though a quadrupole field is turned off.

Motivated by the proposal to simulate para-Bose oscillators in a trapped-ion setup [C. Huerta Alderete and B. M. Rodríguez-Lara, Phys. Rev. A 95, 013820 (2017)], we introduce an overcomplete, nonorthogonal basis for para-Bose Hilbert spaces. The states spanning these bases can be experimentally realized in the trapped-ion simulation via time evolution. The para-Bose states show both nonclassical and semiclassical statistics on their Fock state distribution, asymmetric field quadrature variances, and do not minimize the uncertainty relation for the field quadratures. These properties are analytically controlled by the para-Bose order and the evolution time; both parameters might be feasible for fine tuning in the trapped-ion quantum simulation.

Iso-variance-matrix Laguerre-Gaussian superpositions corresponding to fundamental Laguerre-Gaussian modes are constructed and propagated numerically through atmospheric turbulence. The fact that these superpositions are iso-variance-matrix with the corresponding fundamental Laguerre-Gaussian mode ensures that both the superposition and the corresponding Laguerre-Gaussian mode evolve identically at the level of second moments on free propagation. Consequently they share the same “divergence” and “twist” which remains invariant on free propagation. The robustness of twist associated with such superpositions on passage through atmospheric turbulence is studied. It is found that, for Laguerre-Gaussian superpositions of the form $f\left(\rho \right)exp\left[i\theta \right]$, the twist parameter is generically robust. For other iso-variance-matrix Laguerre-Gaussian superpositions, it is found that the twist is not equally robust on passage through atmospheric turbulence. We illustrate superpositions for which the fluctuation of twist is significantly higher than its corresponding iso-variance-matrix Laguerre-Gaussian mode, on passage through atmospheric turbulence, over short distances.

By introducing loss to one sublattice of dimer chains described by the extended Aubry-André or Harper (AAH) model, we study the topological features including the winding number, the edge states, Chern number, and spectrum of the chain. We find that the parameter region for the system to have real band-gap closing is increased due to the loss, and the average displacement of the single excitation can indicate the topological features of the chain in the presence of loss on one sublattice. The robustness of the zero energy eigenstate against four kinds of disorders are examined. A feasible experimental apparatus based on coupled waveguides to observe the predictions of this work is proposed.

We theoretically study the emission statistics of a weakly nonlinear photonic dimer during coherent oscillations. We show that the phase and population dynamics allow to periodically meet an optimal squeezing condition resulting in a strongly nonclassical emission statistics. By considering an exciton-polariton Josephson junction driven resonantly by a pulsed classical source, we show that a sizable antibunching should emerge in such a semiconductor system where intrinsic nonclassical signatures have remained elusive to date.

We study the fundamental limits of noise spectroscopy using estimation theory, Faraday rotation probing of an atomic spin system, and squeezed light. We find a simple and general expression for the Fisher information, which quantifies the sensitivity to spectral parameters such as resonance frequency and linewidth. For optically detected spin noise spectroscopy, we find that shot noise imposes “local” standard quantum limits for any given probe power and atom number, and also “global” standard quantum limits when the probe power and atom number are taken as free parameters. We confirm these estimation theory results using nondestructive Faraday rotation probing of hot Rb vapor, observing the predicted optima, and finding good quantitative agreement with a first-principles calculation of the spin noise spectra. Finally, we show sensitivity beyond the atom- and photon-number-optimized global standard quantum limit using squeezed light.

We examine optimal paths between initial and final states for diffusive quantum trajectories in continuously monitored pure-state qubits, obtained as extrema of a stochastic path integral. We demonstrate the possibility of “multipaths” in the dynamics of continuously monitored qubit systems, wherein multiple optimal paths travel between the same pre- and postselected states over the same time interval. Optimal paths are expressed as solutions to a Hamiltonian dynamical system. The onset of multipaths may be determined by analyzing the evolution of a Lagrangian manifold in this phase space and is mathematically analogous to the formation of caustics in ray optics or semiclassical physics. Additionally, we develop methods for finding optimal traversal times between states or optimal final states given an initial state and evolution time; both give insight into the measurement dynamics of continuously monitored quantum states. We apply our methods in two systems: a qubit with two noncommuting observables measured simultaneously and a qubit measured in one observable while subject to a Rabi drive. In the two-observable case we find multipaths due to caustics, bounded by a diverging Van Vleck determinant, and their onset time. We also find multipaths generated by paths with different winding numbers around the Bloch sphere in both systems.

We present a comprehensive analysis of longitudinal particle drifting in a standing circularly polarized wave at extreme intensities when quantum radiation reaction (RR) effects should be accounted for. To get an insight into the physics of this phenomenon we made a comparative study considering the RR force in the Landau–Lifshitz or quantum-corrected form, including the case of photon emission stochasticity. Specific features of particle dynamics have a strong impact on spatial structures of the electron-positron ($e^{-}{e}^{+}$) density created in vacuum through quantum electrodynamic (QED) cascades in counterpropagating laser pulses. Three-dimensional particle-in-cell modeling accounting for QED effects confirms realization of different pair plasma structures.

Topological order is now being established as a central criterion for characterizing and classifying ground states of condensed matter systems and complements categorizations based on symmetries. Fractional quantum Hall systems and quantum spin liquids are receiving substantial interest because of their intriguing quantum correlations, their exotic excitations, and prospects for protecting stored quantum information against errors. Here, we show that the Hamiltonian of the central model of this class of systems, the toric code, can be directly implemented as an analog quantum simulator in lattices of superconducting circuits. The four-body interactions, which lie at its heart, are in our concept realized via superconducting quantum interference devices (SQUIDs) that are driven by a suitably oscillating flux bias. All physical qubits and coupling SQUIDs can be individually controlled with high precision. Topologically ordered states can be prepared via an adiabatic ramp of the stabilizer interactions. Strings of qubit operators, including the stabilizers and correlations along noncontractible loops, can be read out via a capacitive coupling to read-out resonators. Moreover, the available single-qubit operations allow to create and propagate elementary excitations of the toric code and to verify their fractional statistics. The architecture we propose allows to implement a large variety of many-body interactions and thus provides a versatile analog quantum simulator for topological order and lattice gauge theories.

An algorithm that initializes a quantum register to a state with a specified energy range is given, corresponding to a quantum implementation of the celebrated Feit-Fleck method. This is performed by introducing a nondeterministic quantum implementation of a standard spectral filtering procedure combined with an apodization technique, allowing for accurate state initialization. It is shown that the implementation requires only two ancilla qubits. A lower bound for the total probability of success of this algorithm is derived, showing that this scheme can be realized using a finite, relatively low number of trials. Assuming the time evolution can be performed efficiently and using a trial state polynomially close to the desired states, it is demonstrated that the number of operations required scales polynomially with the number of qubits. Tradeoffs between accuracy and performance are demonstrated in a simple example: the harmonic oscillator. This algorithm would be useful for the initialization phase of the simulation of quantum systems on digital quantum computers.

Quantum error correction is instrumental in protecting quantum systems from noise in quantum computing and communication settings. Pauli channels can be efficiently simulated and threshold values for Pauli error rates under a variety of error-correcting codes have been obtained. However, realistic quantum systems can undergo noise processes that differ significantly from Pauli noise. In this paper, we present an efficient hard decoding algorithm for optimizing thresholds and lowering failure rates of an error-correcting code under general completely positive and trace-preserving (i.e., Markovian) noise. We use our hard decoding algorithm to study the performance of several error-correcting codes under various non-Pauli noise models by computing threshold values and failure rates for these codes. We compare the performance of our hard decoding algorithm to decoders optimized for depolarizing noise and show improvements in thresholds and reductions in failure rates by several orders of magnitude. Our hard decoding algorithm can also be adapted to take advantage of a code's non-Pauli transversal gates to further suppress noise. For example, we show that using the transversal gates of the 5-qubit code allows arbitrary rotations around certain axes to be perfectly corrected. Furthermore, we show that Pauli twirling can increase or decrease the threshold depending upon the code properties. Lastly, we show that even if the physical noise model differs slightly from the hypothesized noise model used to determine an optimized decoder, failure rates can still be reduced by applying our hard decoding algorithm.

The characterization of entanglement properties in mixed states is important from both a theoretical and a practical point of view. While the estimation of entanglement of bipartite pure states is well established, for mixed states it is a considerably much harder task. The key elements of the mixed-state entanglement theory are given by the exact solutions which sometimes are possible for special states of high symmetry problems. In this paper, we present the exact investigation on the entanglement properties for a five-parameter family of highly symmetric two-qudit mixed states with equal but arbitrary finite local Hilbert space dimension. We achieve this by extensive analysis of various conditions of separability and the entanglement classification with respect to stochastic local operations and classical communication. Furthermore, our results can be used for an arbitrary state by proper application of the proposed twirling operator.

Entanglement criteria for an $n$-partite quantum system with continuous variables are formulated in terms of Rényi entropies. Rényi entropies are widely used as a good information measure due to many nice properties. Derived entanglement criteria are based on several mathematical results such as the Hausdorff-Young inequality, Young's inequality for convolution and its converse. From the historical viewpoint, the formulations of these results with sharp constants were obtained comparatively recently. Using the position and momentum observables of subsystems, one can build two total-system measurements with the following property. For product states, the final density in each global measurement appears as a convolution of $n$ local densities. Hence, restrictions in terms of two Rényi entropies with constrained entropic indices are formulated for $n$-separable states of an $n$-partite quantum system with continuous variables. Experimental results are typically sampled into bins between prescribed discrete points. For these aims, we give appropriate reformulations of the derived entanglement criteria.

Mercury monohalides are promising candidates for electron electric dipole moment searches. This is due to their extremely large values of effective electric fields, besides other attractive experimental features. We have elucidated the theoretical reasons of our previous work. We have also presented a detailed analysis of our calculations, by including the most important of the correlation effects' contributions. We have also analyzed the major contributions to the effective electric field, at the Dirac-Fock level, and identified those atomic orbitals' mixings that contribute significantly to it.

There are major discrepancies between recent $B$-spline $R$-matrix (BSR) and Dirac atomic $R$-matrix code (DARC) calculations regarding electron-impact excitation rates for transitions in ${\mathrm{Mg}}^{4+}$, with claims that the DARC calculations are much more accurate. To identify possible reasons for these discrepancies and to estimate the accuracy of the various results, we carried out independent BSR calculations with the same 86 target states as in the previous calculations, but with a different and more accurate representation of the target structure. We find close agreement with the previous BSR results for the majority of transitions, thereby confirming their accuracy. At the same time the differences with the DARC results are much more pronounced. The discrepancies in the final results for the collision strengths are mainly due to differences in the structure description, specifically the inclusion of correlation effects, and due to the likely occurrence of pseudoresonances. To further check the convergence of the predicted collision rates, we carried out even more extensive calculations involving 316 states of ${\mathrm{Mg}}^{4+}$. Extending the close-coupling expansion results in major corrections for transitions involving the higher-lying states and allows us to assess the likely uncertainties in the existing data sets.

The effects of weak electric fields on resonant energy transfer between ${\mathrm{NH}}_3$ in the $X$ ${{}^{1}A}_1$ ground electronic state and Rydberg He atoms in triplet states with principal quantum numbers $n=36$–41 have been studied in a crossed-beam apparatus. For these values of $n$, electric dipole transitions between the Rydberg states that evolve adiabatically to the $|ns\rangle$ and $|np\rangle$ states in zero electric field can be tuned into resonance with the ground-state inversion transitions in ${\mathrm{NH}}_3$ using electric fields, with energy transfer occurring via Förster resonance. In the experiments the Rydberg He atoms, traveling in pulsed supersonic beams, were prepared by resonant two-photon excitation from the metastable $1s2s{{}^{3}S}_1$ level and crossed an effusive beam of ${\mathrm{NH}}_3$ before being detected by state-selective pulsed-electric-field ionization. The resonant-energy-transfer process was identified by monitoring changes in the ionization signal from the $|ns\rangle$ and $|np\rangle$ Rydberg states for each value of $n$. The electric-field dependence of the experimental data is in good agreement with the results of calculations in which the resonant dipole-dipole coupling between the collision partners was accounted for.